A field-effect transistor (also referred to as a “FET”) is a conventional semiconductor device employable in switch-mode power supplies for use in data processing and telecommunication systems as well as for other applications that use conditioned power for the operation of sensitive electronic circuits. Field-effect transistors have almost universally replaced bipolar transistors previously used for inverters (a type or portion of a power supply) as well as the p-n and Schottky diodes used for rectification. The proliferation of field-effect transistors has, at least in part, been driven by the need to provide highly efficient power supplies with low dc output voltages such as five volts or less at higher current levels. The broad acceptance of field-effect transistors for the aforementioned applications is a consequence of the low forward voltage drop and fast switching speed as well as the low power consumption employed to enable or disable conduction thereof. As a result, the use of field-effect transistors has contributed to compact and efficient power supplies that can be produced at low cost.
As the loads for the power supplies are generally designed with integrated circuits employing shrinking feature sizes for the circuit elements, a need is continually evolving for new designs with lower output voltages (e.g., one volt or less) at higher current levels (e.g., 50 to 100 amperes or more). Present switch-mode power supplies providing input-output circuit isolation (via a transformer) and employing silicon-based field-effect transistors as synchronous rectifiers therein and designed with best current practice are usually operable with switching frequencies only up to several hundred kilohertz (“kHz”) due, at least in part, to the slower switching speeds of the silicon-based field-effect transistors. To accommodate continuing developments in integrated circuit technology, however, power supply switching frequencies above one megahertz (“MHz”) are desirable to reduce the size of magnetic devices and the capacitive filtering elements of the power supply without compromising the power conversion efficiency. In accordance therewith, field-effect transistors with previously unrealizable characteristics are not only being requested, but are necessary to satisfy the aforementioned conditions.
A material of choice for high performance field-effect transistors is a semiconductor with high electron mobility and wide band gap for high breakdown voltage that is capable of being processed with conventional equipment and methods not substantially different from those already developed for silicon and present generations of compound semiconductors. A particularly desirable material is the compound semiconductor known as gallium arsenide (“GaAs”), which has been used for integrated circuits operable at frequencies well above one gigahertz, and which has been used to produce power field-effect transistors with high performance characteristics. An exemplary performance of gallium arsenide in comparison to other semiconductor materials for high performance field-effect transistors is described in “Fundamentals of III-V Devices” by W. Liu, published by John Wiley and Sons (1999), and “Modern GaAs Processing Methods” by R. Williams, published by Artech House (1990), which are incorporated herein by reference.
Additionally, examples of gallium arsenide field-effect transistors employing a controllable vertical channel between a source and drain thereof are provided in the following references, namely, U.S. Pat. No. 5,889,298 entitled “Vertical JFET Field Effect Transistor,” by Plumton, et al., issued on Mar. 30, 1999, U.S. Pat. No. 5,342,795 entitled “Method of Fabricating Power VFET Gate-Refill,” by Yuan, et al., issued on Aug. 30, 1994, U.S. Pat. No. 5,468,661 entitled “Method of Making Power VFET Device,” by Yuan, et al., issued on Nov. 21, 1995, U.S. Pat. No. 5,610,085 entitled “Method of Making a Vertical FET using Epitaxial Overgrowth,” by Yuan, et al., issued on Mar. 11, 1997, and U.S. Pat. No. 5,624,860 entitled “Vertical Field Effect Transistor and Method,” by Plumton, et al., issued on Apr. 29, 1997, which are incorporated herein by reference.
An exemplary gallium arsenide field-effect transistor as generally described in the aforementioned references is illustrated with respect to FIG. 1. The gallium arsenide field-effect transistor includes buried and inter-coupled gate fingers (also referred to as a gate and one of which is designated 110) formed in etched trenches in an n-type doped drain 120 thereby producing vertical channels 130. The gates 110 exhibit a gate length (generally designated “GL”) and the vertical channels 130 provide a channel opening (generally designated “CO”). The trenches are back-filled epitaxially with p-type doped gallium arsenide to form the gates 110. A p+ implant 140 provides a top surface contact to the gates 110, and is made through a further n-type doped epitaxial layer that forms a source 150. External connection to the gates 110 is made through metallized contacts 160 deposited over the heavily doped p+ implants 140. An external source contact 170 and drain contact 180 are made through metal depositions over further heavily doped areas. The structure produces vertical field-effect transistor channels between the gates 110, and provides source and drain contacts 170, 180, respectively, on opposing sides of the die. The device, therefore, operates with vertical carrier flow and the buried gates 110, typically doped opposite from the vertical channels 130, exhibit limited control over a fill profile thereof.
The field-effect transistor as described above, however, does not accommodate a careful doping profiling and epitaxial layer design necessary to produce a modulation-doped channel that can be formed in a lateral channel and that may be employed to substantially reduce the on-resistance of the device. The field-effect transistor described above is not configured with a channel having a strained material, which would significantly improve a mobility of the current carrying carriers and, consequently, a switching speed thereof. This design methodology of constructing such gallium arsenide field-effect transistors has been analyzed in the following references, namely, “Comparison of GaAs and Silicon Synchronous Rectifiers in a 3.3V Out, 50 W DC-DC Converter,” by V. A. Niemela, et al., 27th Annual, IEEE Power Electronics Specialists Conference, Vol. 1, June 1996, pp. 861-867, “10 MHz PWM Converters with GaAs VFETs,” by R. Kollman, et al., IEEE Eleventh Annual Applied Power Electronics Conference and Exposition, Vol. 1, March 1996, pp. 264-269, “A Low On-Resistance High-Current GaAs Power VFET,” by D. L. Plumton, et al., IEEE Electron Device Letters, Vol. 16, Issue 4, April 1995, pp. 142-144, and “RF Power Devices for Wireless Communications,” by C. E. Weitzel, IEEE MTT-S2002, paper TU4B-1, which are incorporated herein by reference. The structure as generally described in the aforementioned references has higher channel and gate resistance per unit die area than the desired resistance and produces a device operable only in the depletion mode, which may limit the applicability of the device in certain situations. Additionally, the devices described in the aforementioned references do not include a body diode. The resulting cost and performance have resulted in limited marketability in view of the presently available silicon-based technology.
Another example of a vertical channel gallium arsenide field-effect transistor is described in “Manufacturable GaAs VFET for Power Switching Applications,” by K. Eisenbeiser, et al., IEEE Electron Device Letters, Vol. 21, No. 4, pp. 144-145 (April 2000), which is incorporated herein by reference. The reference describes forming a source contact on a top surface of the die and a drain contact on the bottom. A vertical channel, however, is still provided in the design proposed by Eisenbeiser, et al. Although economies of manufacture are achieved using implants rather than etching and epitaxial growth as described by Eisenbeiser, et al., possible disadvantages of a channel with a vertical structure limit the switching speed and operation as an enhancement-mode device.
R. Williams describes a GaAs power field-effect transistor in “Modern GaAs Processing Methods,” incorporating both the source and drain contacts on the upper side of a die with a lateral channel and is illustrated with respect to FIG. 2A. The gallium arsenide field-effect transistor acting as a power field-effect transistor includes a gate 205 interposed between a source 210 and drain 215, creating a lateral channel in an n-type doped GaAs layer 220. A depletion region 225 is formed under the gate 205 and a shallower depletion region 230 forms generally under an upper surface of the die as a result of an inherent existence of surface states at the surface of the crystal (i.e., a surface pinning effect). The gallium arsenide field-effect transistor is formed over a semi-insulating GaAs substrate 235. Designing the source and drain contacts on the same surface of the die requires a complicated multilayer metallization process and results in increased die area and reduced chip yield. The aforementioned configuration can also lead to increased inductance, which is of significance in applications for microwave signal amplification. Also, the gallium arsenide field-effect transistor does not include an intrinsic body diode.
Another gallium arsenide field-effect transistor is described by R. Williams at pp. 66-67 of “Modern GaAs Processing Methods,” which is configured with source and drain contacts on opposing sides of the die and a lateral channel, and is illustrated with respect to FIG. 2B. The gallium arsenide field-effect transistor includes a plated air bridge 250 coupled to a source (generally designated “S”) on an upper side of the die. Gates (generally designated “G”) are interposed between drains (generally designated “D”), and are also located on the upper surface of the die. Couplings to the source are brought down to a lower surface of the die by vias 265 that are plated-through holes between the lower surface and the source and are further coupled together by a plated heat sink 280. Although this arrangement can provide low-inductance external connections to the source, it is limited in its ability to provide a low-resistance channel tightly coupled to the lower surface of a highly conductive substrate, or an intrinsic body diode.
The aforementioned design also does not accommodate large drain contacts desirable for a higher power device, which will increase the resistive and inductive losses. This configuration requires a complex air-bridge processing step, which will increase the cost and potentially affect the reliability of the device. For the large gate width field-effect transistor, the parasitic resistance in series with the source will increase due to the long air-bridge connection. This design also is incompatible with today's packaging techniques for higher power devices embodied in a vertical device configuration.
Additionally, U.S. Pat. No. 6,309,918 entitled “Manufacturable GaAs VFET Process,” by Huang, et al, issued on Oct. 30, 2001, is directed toward a vertical FET with source and drain contacts on opposing sides of a die. U.S. Pat. No. 5,956,578 entitled “Method of Fabricating Vertical FET with Schottky Diode,” by Weitzel, et al, issued on Sep. 21, 1999, and U.S. Pat. No. 6,097,046 entitled “Vertical Field Effect Transistor and Diode,” by Plumton, issued on Aug. 1, 2000, are directed toward a vertical FET on a compound semiconductor substrate with an integrated Schottky diode on an upper surface of the die. Neither of the aforementioned references, which are also incorporated herein by reference, provides low on-resistance using a conductivity-enhanced lateral channel.
Still further references such as U.S. Pat. No. 5,068,756 entitled “Integrated Circuit Composed of Group III-V Compound Field Effect and Bipolar Semiconductors,” by Morris, et al., issued on Nov. 26, 1991, and U.S. Pat. No. 5,223,449 entitled “Method of Making an Integrated Circuit Composed of Group III-V Compound Field Effect and Bipolar Semiconductors,” by Morris, et al., issued on Jun. 29, 1993, which are incorporated herein by reference, describe an integration of multiple semiconductor devices on a die including n- and p-channel junction field-effect transistors with a lateral channel. These devices include an isolation trench surrounding the devices, etched to a lower semi-insulating gallium arsenide layer and backfilled with silicon nitride. The aforementioned devices, however, are configured with contacts on a top surface, which restricts their application from low voltage, high current systems wherein efficiency is an important design element.
Low on-resistance compound semiconductor field-effect transistors has been enhanced by the use of a layering structure called modulation doping as described in “Fundamentals of III-V Devices” by W. Liu (p. 323) and “Ultra-High-Speed Modulation-Doped Field-Effect Transistors: A Tutorial Review,” L. D. Nguyen, et al., Proceedings of the IEEE, Vol. 80, No. 4, pp. 494-518 (April 1992), which are incorporated herein by reference and are particularly applicable to devices with a lateral channel. The objective of modulation doping is to avoid electron scattering in the current conducting channel by ionized impurities due to dopants, which increases channel resistivity due to the decreased carrier mobility. Channel conductivity may be modulated by a separate control element such as a gate that controls channel depletion.
The modulation doping technique has been developed for lateral channels using, for instance, gallium arsenide as an undoped, narrower band gap channel and including an adjacent, doped, wider band gap layer of aluminum gallium arsenide, which provides an abundant and controllable source of free carriers to the gallium arsenide channel. Other combinations of compound semiconductors can be used such as an undoped indium gallium arsenide channel and an adjacent, doped aluminum gallium arsenide layer as the source of free carriers. Successful and repeatable applications of modulation doping, which is based on precision formation of a pseudomorphic layer, employs lateral channel arrangements that are not presently available in devices formed with vertical channels.
Power supplies that employ a field-effect transistor as an active switching device frequently rely on an intrinsic body diode to provide a permanently enabled conduction path for current flow in the direction from the source to the drain to safely accommodate inductive circuit elements during a switching operation of the circuit. Alternatively, an external diode may be provided in the circuit as in the case of a typical silicon-based insulated-gate bipolar transistor. In low-voltage applications, low forward voltage drop of the diode is important to maintain circuit efficiency. While a Schottky diode may be employed in parallel with the active switching device, typically the Schottky diode adds cost to the circuit and requires valuable space for the circuit implementation. In addition, avoiding the storage of minority-carrier charge due to current flowing through a p-n-based intrinsic body diode (which can adversely affect the switching operation of the transistor) is preferable in high-frequency switching circuits. The presence of a p-n-based diode generally results in switching losses due to forward and reverse recovery phenomena. These loss mechanisms result, respectively, in increased forward voltage drop immediately after a diode is turned on, and a reverse current flow immediately after a diode is turned off. For examples of diodes integrated into semiconductor devices, see “Design of MOS-Gated Bipolar Transistors with Integral Antiparallel Diode,” by Ajit, J. S., IEEE Electron Device Letters, Vol. 17, Issue 7, July 1996, pp. 344-347, “Optimized Reverse Diode Operation of Power MOSFETs,” by Peppel, M., Weis, B., Conference Record of the 2000 IEEE Industry Applications Conference, Vol. 5, Oct. 8, 2000, pp. 2961-2965, and U.S. Pat. No. 4,967,243, entitled, “Power Transistor Structure with High Speed Integral Antiparallel Schottky Diode,” by B. J. Baliga, et al., issued on Oct. 30, 1990, which are incorporated herein by reference.
Accordingly, a field-effect transistor, especially employing gallium arsenide transistor or other compound semiconductor device, that includes a Schottky diode in parallel with the active switching element, particularly an active switching element with low on-resistance and fast switching speed, would be a highly desirable circuit element, not only for its low forward voltage drop, but also with reduced forward and reverse recovery effects, which are related to its ability to avoid unnecessary charge storage associated with conduction by minority carriers. In addition, an integrated structure incorporating a Schottky diode on the same die with an active switch would reduce the footprint of semiconductor devices in an end product, accommodating thereby an increase in power conversion density and a simplified heat dissipation arrangement.
Considering the limitations as described above, a field-effect transistor design is not presently available for the more severe applications that lie ahead. Accordingly, what is needed in the art is a gallium arsenide and other compound semiconductor devices including a Schottky diode configured for wide acceptance, particularly in low voltage and high current applications, overcoming many of the aforementioned limitations. In accordance therewith, a compact and efficient power supply, particularly for very low voltage and high current loads such as microprocessors, as well as amplifiers for telecommunications and other applications, would benefit from a robust field-effect transistor including a Schottky diode in parallel therewith employable with other devices without incurring unnecessary costs or the need to commit a substantial portion of the physical volume of the end device to the power supply.